The invention relates to a passive sound absorber comprising a cavity opening to the outside in the direction where the acoustic wave in incident via a neck passing through the front wall in order to form a Helmholtz resonator for a first frequency. According to the invention, said absorber further comprises at least one moving element, or wafer, suspended or held by suspensions in a position obstructing said neck in a non-sealed manner.
In addition, the relative stiffness of the suspensions and of the wafer is determined such that the assembly of the wafer and the suspension arms vibrates in a “piston” type resonant mode at a second frequency different from the first frequency, thus producing an absorption for said second frequency or range of frequencies. This second frequency is located between the first frequency and a third frequency which is the frequency of the whole wafer with its suspension when measured in the open air.
Additionally, a hybrid version comprises a coil that is controlled to adjust the acoustic impedance of the absorber.
The invention proposes an acoustic wall comprising a plurality of such absorbers produced by a repetitive structure opening through perforations, each receiving such a wafer.
It also proposes a method for designing and producing such an absorber or wall.
Noise is an important source of noise pollution. Passive noise reduction solutions such as foams are widely applied in most areas.
Passive solutions using Helmholtz resonators are also widely applied, in particular to avoid reflections that can be sources of acoustic resonances. For example, acoustic vases were placed under the stands of Greek or Roman theatres to avoid reflections and improve the acoustics of the building. The size and shape of the vase were adjusted to obtain a resonant system that allowed to suppress acoustic wave reflection in the stands. Nowadays, similar devices are present in the jet engine nacelles.
This system is based on the acoustic resonance of the cavity, which can be described as a “resonant cavity”. The functioning of resonant cavities was conceptualised much later and is now called the “Helmholtz resonator”.
As shown in FIG. 1, the Helmholtz 1 resonator is an open air cavity comparable to an open bottle composed of a neck 11 and a rear volume 10. In the figure, this cavity 10 is enclosed in side walls 19, a bottom wall 18 and a front wall 17, and is only open in the direction A11 via an orifice passing through the front wall 17. This orifice forms a “neck” 11 that has a certain length and thus delimits a volume which is defined by the length L11 of the neck and its opening surface A11, for example, a circular surface that forms a cylindrical neck.
In such a device, the volume of the neck 11 and the rear volume 10 of the cavity are comparable respectively to the mass and the stiffness of a mechanical oscillatory system with one degree of freedom. The absorption is then produced by converting the pressure variation resulting from the acoustic wave into a fluid movement. The energy of the acoustic wave at the resonance frequency of the resonator is then transferred to the resonant system. To attenuate an acoustic wave of a given frequency, the Helmholtz resonator is sized so that its natural frequency is adapted to this frequency to be attenuated according to the following formula:
      f    0    =                    c        0                    2        ⁢        π              ⁢                            A          col                                      L            col                    ⁢                      V            cavité                              where Aneck=π·r2, Lneck and Vcavity are respectively the surface A11 of the opening, the length L11 of the neck 11 and the volume V10 of the rear cavity 10.
Recently, active solutions have been developed that use acoustic transmitters activated and operated according to the acoustic wave to be attenuated for producing destructive interferences that decrease their intensity. However, this type of solution is complex, fragile and expensive.
The choice of a noise reduction device is made according to the cost of the solution to be used, the space requirements and other constraints, such as the operating temperature, as in the case of reducing the noise of plane reactors.
In case of noise compensation in large spaces, such as theatres or traffic halls, the cost of an active absorbing acoustic wall is difficult to predict. Helmholtz resonators or the use of localised active noise compensators make it possible to limit the nuisances specifically related to acoustic resonance modes.
In aircraft reactors, where sound production is very important, civil aviation standards impose increasingly severe restrictions on the emission of aircraft noise. Among all the possible noise reduction solutions, only passive solutions are possible in reactors, due to the very high demands on temperature and vibrations, which can be both acoustic and mechanical.
Instead of using foam, or as an additional solution, cavities tuned as Helmholtz resonators are currently implanted on reactor walls, as shown in FIG. 2. These cavities 10 are made using plates 20 forming a periodic structure in the shape of a honeycomb, as shown in FIG. 2. Such a plate 20 is enclosed between a solid rear plate 28 and a front plate 27. It is pierced with holes 11 opening into cells 10, every one of which constitutes the neck of a resonator 1. This structure allows to adapt the obtained assembly 2 to the shape of the outer wall of the reactor and to ensure its rigidity.
Other passive solutions have been proposed, for instance in document U.S. Pat. No. 8,857,563, which proposes a Helmholtz cavity whose front and/or rear walls are formed of flexible membranes fixed inside the neck, thus deforming one or several walls. These flexible walls sometimes have an orifice and can be equipped with a ballast which allows to change the acoustic response of the walls and of the entire cavity. It has also been proposed to combine Helmholtz cavities with porous materials.
In US 2012/0155688, it is proposed to make a rigid plate of open-cell absorbent which absorbs a first frequency, and to use the flexural stiffness of this plate to absorb a second frequency. In one particular variation, this document also proposes to cut into the plate openings, which can form Helmholtz cavities as known in the state of the art.
One of the purposes of the invention is to overcome the disadvantages of the prior art. This invention seeks improvements, in particular in terms of absorption performance, as well as when it comes to the width and positioning of attenuated frequency ranges. It also seeks to improve the flexibility of implementation and adaptation, that is including the flexibility of design when it comes to the frequencies to be absorbed, for spectrum width and at lower frequencies. Cost, simplicity and reliability as well as resistance to external stresses are also sought after.